A fuel cell has been proposed as a clean, efficient and environmentally responsible power source. Individual fuel cells can be stacked together in series to form a fuel cell stack. The fuel cell stack is capable of supplying a quantity of electricity sufficient to provide power to an electric vehicle.
One type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode on both faces of the membrane-electrolyte. The PEM fuel cell typically includes three basic components: a cathode electrode, an anode electrode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form the membrane-electrode-assembly (MEA).
The MEA is often disposed between porous diffusion media (DM) which facilitate a delivery of gaseous reactants, typically hydrogen and oxygen, for an electrochemical fuel cell reaction. In the fuel cell reaction, hydrogen gas is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit formed therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where oxygen reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
A pair of electrically conductive contact elements or bipolar plates generally sandwich the MEA to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cell's gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the electrodes. Bipolar plates can be assembled by bonding together two unipolar plates having the flow distribution fields formed thereon. Typically, bipolar plates also include inlet and outlet headers which, when aligned in a fuel cell stack, form internal supply and exhaust manifolds for directing the fuel cell's gaseous reactants and liquid coolant to and from, respectively, a plurality of anodes and cathodes.
As is well understood in the art, the membranes within the fuel cell need to have a certain relative humidity to maintain an ionic resistance across the membrane within a desired range to effectively conduct protons. During operation of the fuel cell, moisture from the fuel cell electrochemical reaction and from external humidification may enter the flow channels of the bipolar plates. As moisture is forced along the flow channels by a pressure of reactant gases, the highest concentration of water exists at the outlet regions of the bipolar plates, where reactant gas shear is lowest. Water can accumulate on surfaces in these regions. When the fuel cell is present as part of the fuel cell stack, water vapor is also exhausted to an outlet manifold where the exhausted water vapor condenses on cooler surfaces and drips or runs down the sides of the manifold.
Stagnant water can block flow channels and reduce the overall efficiency of the fuel cell. Liquid water that contacts an edge of the bipolar plate can be pulled into the bipolar plate flow channels by capillary action. Bipolar plates having a hydrophilic treatment are particularly susceptive to the capillary action of liquid water that accumulates at the edge of the bipolar plates. A high degree of water accumulation or stagnation can also lead to fuel cell failure, particularly following a shut-down period under freezing ambient conditions where the accumulated water turns to ice. Both accumulated water and ice may cause gas starvation. Gas starvation is know to result in carbon corrosion when the starved fuel cell is one of a number of fuel cells in the fuel cell stack having an electrical load applied thereto.
A known strategy for militating against water stagnation includes high flow purging of the fuel cell stack to force accumulated water from the fuel cells. Typical fuel cell stacks have also employed supplemental heating, for example, through electrical resistance at start-up to melt ice having formed during a shut-down in freezing conditions. These methods require active controls, however, and undesirably add to a complexity and cost of a system including the fuel cell.
There is a continuing need for a water management feature that transports accumulating water away from fuel cells in a fuel cell stack. Desirably, the feature is passive and improves fuel cell performance, particularly after a shut-down period under freezing ambient conditions.